This invention generally relates to camera lens barrels and, more particularly, to camera lens barrels suitable for use in flash photography.
In a known camera lens barrel, flash guide numbers are marked either on the distance adjusting ring or the diaphragm value adjusting ring. The distance adjusting ring, which contains distance gradations on its outer circumference, and the diaphragm value adjusting ring, which contains diaphragm value graduations on its outer circumference, are so arranged that when the value of each ring is increased by the .sqroot.2, these two rings are rotated by an equal rotation angle. For example, if the distance value is changed from 5m to 7m (5 .times..sqroot. 2 .congruent. 7), the diaphragm value is changed from 5.6 to 4 (4 .times..sqroot. 2 .congruent. 5.6). A clutch member mounted in said camera lens barrel is so arranged that, in flash photography, it connects the distance adjusting ring and the diaphragm value adjusting ring at a relative angular position determined by the flash guide number. In such a lens barrel, it is known that the use of a cam helicoid for the purpose of shifting the lens can be used. The use of the cam helicoid eliminates a lever, string and cam plate which are indispensible in lens barrels that use a screw helicoid arrangement to affect the distance adjustment and diaphragm adjustment in association with each other. The lens barrel of the present invention employs a cam helicoid arrangement for shifting the lens.
For the convenience of distance adjustment and TTL photography, single lens reflex cameras generally employ a preset automatic diaphragm device. Such devices allow the viewfinder image to be more readily visible because the diaphragm is kept in its full-open position during initial distance adjustment. The depth of field for a given diaphragm value is determined through the viewfinder by stopping down the diaphragm to the given value. This action is normally performed by operating a depth of field determining lever arranged either on the side of the camera or lens. In flash photography, however, the subject is dark and it is impractical to determine the depth of field through the viewfinder.
In a conventional camera lens barrel of screw helicoid-type, there is a linear relationship between the rotation angle of the distance adjusting ring and the lens shifting quality. Accordingly, if a given diaphragm value is expressed by F, the lead of the screw helicoid by L and the resolving power of the eye by .delta., then the indicated position H of the depth of field's gradation at the given diaphragm value may be expressed by the following equation: EQU H = .+-.360.multidot..delta. F)/L (1)
(where .delta. normally equals the focal length divided by 1500).
In a lens barrel of cam helicoid-type, however, the relationship between the rotation angle of the distance adjusting ring and the lens shifting quality is not the same. Unlike a lens barrel of the screw helicoid-type, it is impossible in a lens barrel of the cam helicoid type to indicate the depth of field's gradation for a given diaphragm value. Thus, the use of such a lens in flash photography results in the encountering of a difficulty in determining the depth of field.
Therefore, it is an object of this invention to provide a new and improved camera lens barrel suitable for use in flash photography.
It is another object of this invention to provide a new and improved clutch arrangement suitable for coupling a distance adjusting ring to a diaphragm value adjusting ring during flash photography.
It is yet another object of this invention to provide a new and improved arrangement for coupling a distance adjusting ring to a diaphragm value adjusting ring in a camera lens barrel that employs a cam helicoid for shifting the lens.